The Casting Director will be used primarily for smaller roles this time around I would have thought...I doubt they are using a CD for Bilbo and main dwarfs for example but the Casting directors will issue breakdowns for the smaller dwarf roles and then the elves and main goblins etc...

on LOTR I think all of the characters had a breakdown issued for them but I can't see that happening now since there will already be front-runners as you say and realistically I think Bilbo now knows who he is...

Then very small roles though (actors only on set for a few days) will probs all be cast from NZ or at a push Oz...purely for the logistical and cost reasons of having to fly people over from US or UK for only a few days, and sorting work visa's etc...

My agent seems to know who the London CD is but I'm waiting confirmation before I say it...just in case it's wrong and I make a tit of myself...haha